On 20 September 1917 east of Ypres, Belgium, when all the other officers of his company and all but one in the leading company had become casualties, Second Lieutenant Colvin took command of both companies and led them forward under heavy fire with great success. He went with only two men to a dug-out, when he left the men on top, entered it alone and brought out 14 prisoners. He then proceeded to clear other dug-outs, alone or with only one man, capturing machine-guns, killing some of the enemy and taking a large number of prisoners.